Interpretive Summary: The neonatal stage is a critical time in the life of a pig, when they are prone to become sick or weak. This is the stage at which most euthanasia procedures are required if the pig is judged unable to recover. Any euthanasia method should be humane, practical, economical and socially acceptable to be universally accepted. Blunt force trauma is widely used to euthanize neonatal piglets, by administrating a blow to the head which results in severe brain damage given the thin skull bones and results in immediate loss of consciousness. Data support that blunt force trauma is humane when carried out correctly, but most people find it visually difficult to accept. Furthermore, it can be emotionally disturbing for the stockperson. The use of carbon dioxide (CO2) is often recommended. At the same time, carbon dioxide is criticized as being aversive to, causing a profound sense of breathlessness. Chemically inert gases such as nitrogen, argon and nitrous oxide possess anesthetic properties without imparting any sense of breathlessness. Data from these studies support the idea that exposure to a mixture of nitrous oxide and oxygen before using carbon dioxide may be a more humane method of euthanasia than exposure to carbon dioxide alone. This research sought to: 1) identify a method of scientifically determining if piglets find a gas aversive, using an approach-avoidance test which relies on the piglet’s own perspective (Experiments 1 and 2), and 2) test different gas mixtures to determine if they are effective and humane for neonatal piglet euthanasia, using a two-step anesthesia-euthanasia procedure (Experiment 3). We hypothesized that nitrous oxide, and alternatively argon and nitrogen, were less aversive than carbon dioxide and would induce anesthesia prior to using carbon dioxide to complete the euthanasia procedure. We found that nitrous oxide in oxygen appeared to be less aversive than nitrous oxide, nitrogen, or argon all combined with low (30%) concentrations of carbon dioxide or 90% carbon dioxide by itself. Our study is the first to investigate the use of nitrous oxide at sufficiently high concentrations to cause anesthesia. Nitrous oxide, commonly referred to as laughing gas, has been widely used in human surgery and dental offices for its pain-relieving, sedative and anxiolytic effects. It is cheap, non-flammable, non-explosive, legally accessible and not classified as a drug in the U.S., and already commonly used in the food industry as a propellant for food products. Development of its use into an automated procedure will allow producers to implement it with little effort. Thus its use as an anesthetic/euthanasia agent may prove to be affordable, feasible and more humane than other alternatives.

Technical Abstract:
The search for alternative methods to euthanize piglets is critical to address the public’s concern that current methods are not acceptable. Scientific evidence support that blunt force trauma is humane when carried out correctly, but most people find it visually difficult to accept. The use of carbon dioxide is often recommended, at the same time it is criticized as being aversive. This research sought to: 1) identify a method of scientifically determining if piglets find a gas aversive, using an approach-avoidance test which relies on the piglet’s perspective, and 2) test different gas mixtures to determine if they are effective and humane for neonatal piglet euthanasia. Pigs were allowed to walk freely between one chamber filled with air and another chamber either gradually filled with gas (Experiment 1) or pre-filled with gas (Experiment 2). Experiment 1 tested carbon dioxide, CO2 (90%) and air (10%); nitrous oxide, N2O (60%) and CO2 (30%); argon, Ar (60%) and CO2 (30%); and nitrogen, N2 (60%) and CO2 (30%). Since piglets had to be removed when they became panicked, the test was shortest (P < 0.01) for the pigs in the CO2 treatment compared to pigs in the N2O/CO2, Ar/CO2, and N2/CO2 treatments, 3.1 ± 0.2, 8.5 ± 0.6, 9.6 ± 0.4, and 9.9 ± 0.1 min, respectively. Nonetheless, all gas mixtures adversely affected the pigs, causing the pigs to leave the test chamber. In Experiment 2, piglets were allowed to enter a chamber pre-filled with N2/CO2 or N2O/CO2 (both 60%/30%). Pigs exposed to the pre-fill chambers entered a state of panic in less than 20 s, much faster in comparison to the gradual fill method, which support than this method was more aversive. In Experiment 3, piglets were euthanized using a 2-step procedure. Pigs were first placed in a gradual fill chamber with 1 of 4 gas mixtures: 90% CO2, N2/CO2, N2O/CO2 or N2O/O2 (the last three mixtures at 60%/30%) followed by placement into a 90% CO2 pre-fill chamber when the pigs became panicked or were anesthetized. All three gas treatments that contained CO2 killed pigs more quickly than N2O/O2 (P < 0.05). However, N2O/O2 was the only treatment that anesthetized the pigs instead of causing squeals or panic, although requiring about 12 min longer. Although longer, a 2-step procedure in which pigs are anesthetized with a mixture of N2O and O2 prior to being euthanized by immersion in CO2 may prove to be more humane than CO2 alone.